Stucco Myth 1In our work as forensic architects and engineers, we are regularly involved in litigation over stucco failures, including hotels and high-rise condo complexes. (For this article, ‘stucco’ refers to traditional portland cement plaster direct-applied to a masonry substrate, rather than using lath.)

Myths abound around stucco cracking. In truth, it is not abnormal to have some cracking with stucco, much of which can be relatively harmless. The key is paying attention to the types of cracks, and minimizing any significant issues that might lead to actual failure, including debonding, water intrusion, and mold problems.

 

Myth #1: Stucco on lath over CMU/concrete is superior to direct-applied.

It is a myth stucco on lath over CMU/concrete is superior to direct-applied, as illustrated by the cracks in this hotel wall. Despite protestations by the stucco sub, stucco was installed on lath over concrete per the architect’s direction.In our work as forensic architects and engineers, we are regularly involved in litigation over stucco failures, including hotels and high-rise condo complexes. (For this article, ‘stucco’ refers to traditional portland cement plaster direct-applied to a masonry substrate, rather than using lath.)

Myths abound around stucco cracking. In truth, it is not abnormal to have some cracking with stucco, much of which can be relatively harmless. The key is paying attention to the types of cracks, and minimizing any significant issues that might lead to actual failure, including debonding, water intrusion, and mold problems.

Many believe direct-applied stucco is more prone to cracking, and that stucco easily falls off if not adhered to lath. Stucco on lath is often preferred by architects and owners because it can create a drainage plane behind the cladding, which is thought to be needed to remove water.

However, direct-applied to masonry is a better approach if the substrate has been properly prepared. It creates a mass wall, minimizing potential for water intrusion as long as changes in substrate are addressed (unless the masonry itself cracks). A mass wall does not need a drainage plane. We often see major cracking where lath was improperly used in lieu of direct applied stucco.

In his Techniques and Comments newsletter, the late stucco expert John Bucholtz explained it this way:

Occasionally someone will specify that metal lath be applied to block. It’s an erroneous specification for new block. Old block—that’s a different animal. Inclusion of metal lath will assure no delamination, but it makes up for it by developing plenty of cracks.

Bucholtz said lath may be desirable for old concrete masonry units (CMUs) because blocks may have been painted or are dirty and weathered, limiting good bonding of the stucco.

Authors: Richard Scott, AIA, NCARB, LEED AP, is a senior forensic architect, Charles Allen, AIA, is a forensic architect, Steve Gleason, PE, is a senior forensic engineer.

Adapted from an originally published article in Construction Specifier. Source URL: http://www.constructionspecifier.com/stucco-myths-and-facts/ May 4th, 2016.